Capacitive sensing for paper tray

ABSTRACT

A capacitive sensing system based on projected self-capacitance is suitable for use in printing systems/products to sense paper tray status. In example embodiments, a capacitive sensing system is adapted for sensing the condition/characteristics of paper in the paper tray, such as paper size, stack height and page count and paper dielectric. The capacitive sensing system can be configured with one or more shielded capacitive sensors incorporated into the paper tray, and oriented relative to the paper according to the paper condition/characteristic sensed.

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is claimed under USC§119(e) to: (a) U.S. Provisional Application 61/915,036 (Texas Instruments docket TI-74604PS), filed 12 Dec. 2013, and (b) U.S. Provisional Application 61/932,394 (Texas Instruments docket TI-74604PS1), filed 28, Jan., 2014.

BACKGROUND

1. Technical Field

This Patent Document relates generally to printing systems/products that include sheet feeding from a paper tray.

2. Related Art

Printing systems/products that provide printed (paper) output typically include a sheet-feeding apparatus. The sheet-feeding apparatus feeds paper to a printing apparatus from a paper tray.

These printing systems/products commonly include various mechanical or electronic mechanisms to determine the condition or characteristics of the paper in the tray. For example, a paper tray sensing mechanism can be used to determine the paper quantity and paper size.

BRIEF SUMMARY

This Brief Summary is provided as a general introduction to the Disclosure provided by the Detailed Description and Figures, summarizing some aspects and features of the disclosed invention. It is not a complete overview of the Disclosure, and should not be interpreted as identifying key elements or features of the invention, or otherwise characterizing or delimiting the scope of the invention disclosed in this Patent Document.

The Disclosure describes apparatus and methods for capacitive sensing for paper tray status (paper condition/characteristics), such as paper size, stack height and page count, and paper dielectric.

According to aspects of the Disclosure, a method for measuring paper characteristics of paper within a paper tray using capacitive sensing can be used with a paper tray that includes at least two capacitive sensors with respective capacitive electrodes C_(IN1) and C_(IN2), oriented relative to the paper in the paper tray such that paper covers C_(IN1), and partially covers C_(IN2) in the width dimension. The method can include: (a) measuring a capacitance C_(A1)=C_(IN1) to ground; (b) measuring a capacitance C_(A2)=C_(IN2) to ground; and (c) determining C_(DIFF) _(—) _(W)=C_(A1)−C_(A2); and determining paper width based on C_(DIFF) _(—) _(W) corresponding to a percentage of C_(IN2) covered by paper.

According to other aspects of the Disclosure, a method of measuring paper characteristics for paper within a paper tray using capacitive sensing can be used with a paper tray that includes at least one capacitive sensor with a capacitive electrode C_(IN1), oriented relative to the paper in the paper tray such that the paper covers C_(IN1). The method includes: (a) measuring C_(A1)=C_(IN1) to ground; and (b) determining paper stack height based on C_(A1). The method can further include calibrating for tray thickness, sensor size/position and sensor ground plane position, based on a capacitive measurement C_(A0)=C_(IN1) to ground (with no paper present). The method can further include determining page count by first:

(a) measuring capacitance CA1,0

${{CA}\; 1,0} = {\left( {{CAIR}^{- 1} + {CPAPER}^{- 1}} \right)^{- 1} = \left\lbrack {\frac{1}{{CA}\; 0} + {\frac{dp}{ɛ\; 0\; A}\left( {\frac{1}{ɛ\; P} - \frac{1}{ɛ\; A}} \right)^{- 1}}} \right\rbrack}$ where ${CA}_{0} = {k*ɛ_{0}*ɛ_{A}*\frac{A}{d}}$ ${CAIR} = {k*ɛ_{0}*ɛ_{A}*\frac{A}{d - {dp}}}$ ${CPAPER} = {k*ɛ_{0}*ɛ_{P}*\frac{A}{dp}}$

and where ∈_(A) is the dielectric constant of the air, k accounts for fringing, A is sensor area, and

(b) determining d_(p) as the total thickness of paper between sensor C_(IN1) and ground (total stack height), where d_(a) is the total thickness of air between the sensor and ground, so that d=d_(p)+d_(a)→d_(a)=d−d_(p).

And, then, feeding one page of paper, and:

(c) measuring capacitance C_(A1,1)

${{CA}\; 1},{1 = \left\lbrack {\frac{1}{{CA}\; 0} + {\frac{\left( {{dp} - {d\; 1\; {page}}} \right)}{ɛ\; 0\; A}\left( {\frac{1}{ɛ\; P} - \frac{1}{ɛ\; A}} \right)}} \right\rbrack^{- 1}}$

(d) determining d_(1page); and

(e) determining page count=dp/(d1page).

Other aspects and features of the invention claimed in this Patent Document will be apparent to those skilled in the art from the following Disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example functional illustration of a capacitive sensing system based on projected self-capacitance, adaptable for use in printing systems/products to sense the condition/characteristics of paper in the paper tray, such as paper size and paper stack height/page count, and to determine paper type (paper dielectric).

FIGS. 2A, 2B and 2C illustrate an example embodiment of a paper tray with capacitive sensing for measuring paper size/width, including: (2A/2B) top and side views illustrating a paper tray incorporating a capacitive sensor, including capacitive electrodes C_(IN1) and C_(IN2), and (2C) example capacitive sensing results.

FIGS. 3A, 3B and 3C illustrate an example embodiment of a paper tray with capacitive sensing for measuring paper stack height/page count, including: (3A/3B) top and side views illustrating a paper tray incorporating a capacitive sensor, including capacitive electrode C_(IN1), and (3C) example capacitive sensing results.

FIG. 4 illustrates an example embodiment of a paper tray with a capacitive sensing for measuring paper dielectric, including example inter-digitated (co-planar) capacitive and ground electrodes E1 and G1.

DETAILED DESCRIPTION

This Description and the Figures disclose example embodiments and applications that illustrate various features and advantages of a capacitive system for sensing paper tray status.

In brief overview, a capacitive sensing system is based on projected self-capacitance. In example embodiments, the capacitive sensing system can be configured with one or more shielded capacitive sensors incorporated into the paper tray, and oriented relative to the paper according to the paper condition/characteristic sensed.

FIG. 1 is an example functional illustration of a capacitive sensing system 100 suitable for use in printing systems/products, and in particular, for use in sensing paper tray status. More particularly, capacitive sensing system 100 is adapted to embodiments of the invention used to sense the condition/characteristics of paper in the paper tray, such as paper size, stack height and page count, and paper dielectric.

Capacitive sensing system 100 includes a capacitive sensor 110 and capacitance acquisition/conversion 130 formed by a capacitance-to-digital conversion (CDC) unit 150, and a data processor 170.

In example embodiments, capacitive sensor 110 is adapted for incorporation into a paper tray, and configured for capacitive sensing of condition/characteristics of paper 120. The capacitive sensor 110 need not be co-located with the CDC unit 150, but to reduce the effects of parasitic capacitance, CDC 150 is preferably located as close as possible to capacitive sensor 110.

Capacitive sensing system 100 is configured for capacitive sensing based on projected self-capacitance. Capacitive sensor 110 includes a sensor electrode 111 and a driven sensor shield 113, separately coupled to CDC 150 (Acquisition Channel input CH and Shield Excitation/Driver output SHIELD).

Capacitive sensor 110 includes a driven sensor shield 113, also coupled to a shield driver in CDC 150. Sensor shield 113 is disposed over, and insulated from, sensor electrode 111. Shield drive can be provided synchronously with sensor excitation frequency, and can be used to focus sensing direction, and to counteract parasitic capacitance.

CDC 150 acquires capacitance measurements from capacitive sensor 110, and converts these capacitance measurements to digital sensor data representative of paper condition/characteristics. The CDC sensor data can be input to data processor 170, and processed to provide paper tray status information.

FIGS. 2A, 2B and 2C illustrate an example embodiment of a paper tray 201 with capacitive sensing adapted for measuring paper size/width, including a capacitive sensor with capacitive electrodes C_(IN1) and C_(IN2) (211 and 212). The capacitive sensing system includes a capacitive sensor structure with (shielded) capacitive electrodes C_(IN1) and C_(IN2) configured for capacitance measurements of paper size/width based on projected self-capacitance.

FIGS. 2A and 2B are top and side views that functionally illustrate an example arrangement for the elements of the capacitive sensor incorporated with a paper tray 201, and in particular, the placement of the capacitive electrodes C_(IN1) and C_(IN2) relative to the paper 220, which is aligned within the tray at 203. Specifically, capacitive sensor/electrode C_(IN1) is positioned so that it is covered by paper 220 in tray 201, and capacitive sensor/electrode C_(IN2) is positioned so that it is partially covered in the width dimension by paper 220 in tray 201.

Referring particularly to FIG. 2B, a capacitive sensor includes capacitive electrodes C_(IN1) and C_(IN2) (211 and 212), shield 213 and insulator 214, integrated or mounted within tray 201. A ground plane 219 is spaced from the capacitive sensors C_(IN1) and C_(IN2) in the projection direction. As illustrated, ground plane 219 is on the bottom side of tray 201—alternatively, the ground plane can be located within the tray, adjacent paper 201.

For paper size/width measurement, C_(IN1) measurements are used to calibrate for the type of paper, and combined C_(IN1) and C_(IN2) measurements are used to determine paper size/width. Assumptions for the example embodiment are:

C_(IN1) is completely covered by paper

C_(IN1) sensor is substantially identical to C_(IN2) sensor

If C_(IN1) and C_(IN2) are not identical, C_(IN2) can be calibrated.

An example methodology for determining paper size/width based on capacitive sensing involves first calibrating for tray thickness and sensor size/position based on a capacitive measurement C_(A0)=C_(IN1) to ground (with no paper present).

For paper size/width measurement operations, with paper present, C_(IN1) and C_(IN2) measurements are captured:

C_(A1)=C_(IN1) to ground

C_(A2)=C_(IN2) to ground

C_(DIFF)=C_(A1)−C_(A2)

Paper width can be determined from a percentage of C_(IN2) covered by paper, as represented by C_(DIFF).

An example methodology for determining paper size/width based on capacitive sensing, includes

${{CA}_{0} = {k*ɛ_{0}*ɛ_{A}*\frac{a*b}{d}}},$

∈_(A) is the dielectric constant of the air, k accounts for fringing

${CA}_{1} = {k*ɛ_{0}*ɛ_{P}*\frac{a*b}{d}}$

∈_(P) is the dielectric constant of the paper

${CDIFF} = {{k*ɛ_{0}*ɛ_{P}*\frac{a\; 1*b}{d}} - {k*ɛ_{0}*ɛ_{A}*\frac{a\; 2*b}{d}}}$

a1 is the length of the portion of C_(IN2) electrode covered by the paper

a2 is the length of the portion of C_(IN2) electrode not covered by the paper

a=a1+a2

CDIFF=CA _(—)1*(1−a1/a)CA _(—)0*a2/a

a1=a−a2 (since a=a1+a2)

a2=(a*CDIFF)/(CA_1−CA_0)

This methodology for determining paper size/width is independent of the dielectric of paper ∈_(P.)

Referring to FIG. 2B, the capacitive sensor (shield 213, insulator 214, electrodes C_(IN1)/C_(IN2)) is mounted above tray 201 and paper 220, so that the sensing field projects through the paper 220 toward the ground plane 219. In an alternate configuration, the sensor can be mounted on the tray, oriented 180 degrees to the orientation illustrated, with the sensor electrodes C_(IN1)/C_(IN2) adjacent paper 220.

A third capacitive sensor/electrode C_(IN3) can be used to measure paper length using a similar method. Capacitive sensor/electrode C_(IN3) can be positioned so that it is partially covered in the length dimension by paper 220 in tray 201 (substantially as illustrated for C_(IN2) for the width dimension).

FIG. 2C provides example measurement results based on the following parameters:

C _(IN1) and C _(IN2) electrode length is a=104 mm (a1+a2)

Letter paper is approximately 6 mm wider than A4

Cdiff_nopaper accounts for non-identical electrodes and is subtracted from CDIFF.

Example design modifications for the configuration of the capacitive electrodes C_(IN1) and C_(IN2) include, in addition to size/perimeter, different shapes/profiles, such as spiral.

FIGS. 3A, 3B and 3C illustrate an example embodiment of a paper tray with capacitive sensing for measuring paper stack height and page count. The capacitive sensing system includes a capacitive sensor structure with a (shielded) capacitive electrode C_(IN1) (311) configured for capacitance measurements of paper stack height/page count based on projected self-capacitance.

FIGS. 3A and 3B are top and side views that functionally illustrate an example arrangement for the elements of the capacitive sensor incorporated with a paper tray 301, and in particular, the placement of the capacitive electrode C_(IN1) relative to the paper 320, which is aligned within the tray at 303. Specifically, capacitive sensor/electrode C_(IN1) is positioned so that it is covered by paper 220 in tray 201.

Referring particularly to FIG. 3B, a capacitive sensor includes a capacitive electrode C_(IN1) (311), shield 313 and insulator 314, integrated or mounted within tray 301. A ground plane 319 is spaced from the capacitive sensor C_(IN1) in the projection direction. As illustrated, ground plane 319 and bottom side of tray 301—alternatively, the ground plane can be located within the tray, adjacent paper 301.

An example methodology for sensing paper stack height involves first calibrating for tray thickness and sensor size/position based on a capacitive measurement C_(A0)=C_(IN1) to ground with no paper present.

For stack height measurement operation, with paper present, the C_(IN1) measurement is captured: C_(A1)=C_(IN1) to ground, which is proportional to a total thickness of the paper between CIN1 and ground, i.e., total paper stack height.

Page count can be determined from an initial sheet feed. An example methodology for calculating the number of pages in the paper stack includes: (a) feed one paper sheet, and determine from capacitive measurements the change in stack height, so that (b) page count=previous stack height/change in stack height.

An example methodology for determining page count includes two capacitive measurements. First, measure capacitance CA1,0:

$\begin{matrix} {{{CA}\; 1},{0 = \left( {{CAIR}^{- 1} + {CPAPER}^{- 1}} \right)^{- 1}}} \\ {= \left\lbrack {\frac{1}{{CA}\; 0} + {\frac{dp}{ɛ\; 0\; A}\left( {\frac{1}{ɛ\; P} - \frac{1}{ɛ\; A}} \right)}} \right\rbrack^{- 1}} \end{matrix}$

where d_(p) is the total thickness of paper between sensor CIN1 and ground; and where:

${CA}_{0} = {k*ɛ_{0}*ɛ_{A}*\frac{A}{d}}$ ${CAIR} = {k*ɛ_{0}*ɛ_{A}*\frac{A}{d - {dp}}}$ ${CPAPER} = {k*ɛ_{0}*ɛ_{P}*\frac{A}{dp}}$

and where

d_(a) is the total thickness of air between the sensor and ground

d=d_(p)+d_(a)

d_(a)=d−d_(p)

∈_(A) is the dielectric constant of the air

k accounts for fringing

A is sensor area.

Then feed one page of paper, and measure capacitance C_(A1,1)

${{CA}\; 1},{1 = \left\lbrack {\frac{1}{{CA}\; 0} + {\frac{\left( {{dp} + {d\; 1{page}}} \right)}{ɛ\; 0\; A}\left( {\frac{1}{ɛ\; P} - \frac{1}{ɛ\; A}} \right)}} \right\rbrack^{- 1}}$

where d_(1page) is the thickness of one sheet of paper.

Capacitance CA1,0 can be used to determine d_(p) as the total thickness of paper between sensor CIN1 and ground (total stack height), and CA1,1 can be used to determine d_(1page) is the thickness of one sheet of paper. Then page count can be determined as: page count=dp/(d1page).

This initial sheet-feed methodology, which provides sheet thickness d_(1page) does not require prior knowledge of the dielectric constant of the paper ∈_(P.)

The dielectric constant of the paper ∈_(P) (paper type) can be determined from the above measurement for paper stack height and page count, including the determination of sheet thickness d_(1page), which enables computation of the average dielectric ∈_(eff) between C_(IN1) and GND.

Average dielectric ∈_(eff) and the dielectric constant of the paper ∈_(P) are related by:

$ɛ_{eff} = \frac{1}{{{nw}/{d\left( {{1/ɛ_{paper}} - {1/ɛ_{air}}} \right)}} + {1/ɛ_{air}}}$

where n=number of pages, w=sheet thickness (d_(1page)), so that stack height is (nw); is the stack height (d_(p)), and (d) is the distance between the capacitive electrode C_(IN1) and GND.

Based on the known values:

Distance between CIN1 and GND, (d)

Average dielectric between CIN1 and GND, ∈_(eff)

Number of pages in the stack (n), d_(p)/(d_(1page))

Thickness of a single sheet of paper (w) paper dielectric ∈_(paper) (paper type) can be determined from:

${1/ɛ_{{pape}\underset{\_}{\underset{\_}{r}}}}\frac{{d\; ɛ_{air}} + {\left( {{nw} - d} \right)ɛ_{eff}}}{{nw}\mspace{11mu} ɛ_{eff}ɛ_{air}}$

FIG. 3C provides example page count measurement results, comparing expected to actual results.

As an alternate embodiment for determining page count using an initial sheet feed (i.e., to determine sheet thickness d_(1page)), the dielectric of the paper ∈_(paper) can be capacitively sensed, and page count determined if sheet thickness is known, or assumed.

FIG. 4 illustrates an example embodiment of a capacitive sensor arrangement adapted for measuring paper stack height/page count based on capacitively sensing paper dielectric. Two capacitive sensors are used. A capacitive sensor/electrode C_(IN1) (411) is used as in the embodiment in FIGS. 3A/B. An inter-digitated (co-planar) capacitive sensor with capacitive/ground electrodes E1/G1 is used for measuring paper dielectric ∈_(paper.)

Capacitive electrode C_(IN1) is operable for the capacitive measurement C_(A1,0) as described in connection with FIGS. 3A/3B—the capacitive measurement C_(A1,1) after an initial sheet feed (to obtain d_(1page)), need not be taken. The C_(IN1) ground plane 419 can be disposed relative to C_(IN1) as described above in connection with FIGS. 3A/3B.

For capacitive sensing in connection with determining paper dielectric ∈_(paper), the inter-digitated (co-planar) capacitive/ground electrodes E1/G1 are disposed on the interior surface of a paper tray, for example at the alignment corner (in FIG. 3A, at 303). The sensor/ground electrodes E1/G1 are configured and oriented in an inter-digitated co-planar arrangement, preferably so that the projected fringe fields are confined to the typical thickness of one sheet of paper (approximately 100 microns).

Capacitance CD is measured with and without paper, to obtain the paper dielectric ∈_(paper.)

For this embodiment, which does not require an initial sheet feed to determine paper thickness (i.e., d_(1page)), page count requires knowledge of paper dielectric ∈_(paper) and paper sheet thickness. Paper thickness can be determined by, for example, separate input, or based on assumption, for example, a standard paper thickness of approximately 100 microns.

The Disclosure provided by this Description and the Figures sets forth example embodiments and applications, including associated operations and methods, that illustrate various aspects and features of the invention. Known circuits, functions and operations are not described in detail to avoid unnecessarily obscuring the principles and features of the invention. These example embodiments and applications can be used by those skilled in the art as a basis for design modifications, substitutions and alternatives to construct other embodiments, including adaptations for other applications. Accordingly, this Description does not limit the scope of the invention, which is defined by the Claims. 

1. A method of measuring paper characteristics for paper within a paper tray using capacitive sensing, the paper tray including at least two capacitive sensors with respective capacitive electrodes C_(IN1) and C_(IN2), oriented relative to the paper in the paper tray such that the paper covers C_(IN1), and partially covers C_(IN2) in the width dimension, the method comprising measuring a capacitance C_(A1)=C_(IN1) to ground; measuring a capacitance C_(A2)=C_(IN2) to ground; and determining C_(DIFF) _(—) _(W)=C_(A1)−C_(A2;) determining paper width based on C_(DIFF) _(—) _(W) corresponding to a percentage of C_(IN2) covered by paper.
 2. The method of claim 1, further comprising calibrating for tray thickness, sensor size/position and sensor ground plane position, based on a capacitive measurement C_(A0)=C_(IN1) to ground (with no paper present).
 3. The method of claim 1, further including a third capacitive sensor C_(IN3) oriented such that the paper partially covers the third capacitive sensor in the length dimension, and further comprising: measuring a capacitance C_(A1)=C_(IN1) to ground; measuring a capacitance C_(A3)=C_(IN3) to ground; and determining C_(DIFF) _(—) _(L)=C_(A1)−C_(A3;) determining paper length based on C_(DIFF) _(—) _(L) corresponding to a percentage of C_(IN3) covered by paper.
 4. A method of measuring paper characteristics for paper within a paper tray using capacitive sensing, the paper tray including at least one capacitive sensor with a capacitive electrode C_(IN1), oriented relative to the paper in the paper tray such that the paper covers C_(IN1), the method comprising: measuring C_(A1)=C_(IN1) to ground; determining paper stack height based on C_(A1).
 5. The method of claim 4, further comprising calibrating for tray thickness, sensor size/position and sensor ground plane position, based on a capacitive measurement C_(A0)=C_(IN1) to ground (with no paper present).
 6. The method of claim 4, further comprising determining page count by: measuring capacitance CA1,0 ${{CA}\; 1},{0 = {\left( {{CAIR}^{- 1} + {CPAPER}^{- 1}} \right)^{- 1} = \left\lbrack {\frac{1}{{CA}\; 0} + {\frac{dp}{ɛ\; 0\; A}\left( {\frac{1}{ɛ\; P} - \frac{1}{ɛ\; A}} \right)}} \right\rbrack^{- 1}}}$ where ${CA}_{0} = {k*ɛ_{0}*ɛ_{A}*\frac{A}{d}}$ ${CAIR} = {k*ɛ_{0}*ɛ_{A}*\frac{A}{d - {dp}}}$ ${CPAPER} = {k*ɛ_{0}*ɛ_{P}*\frac{A}{dp}}$ and where ∈_(A) is the dielectric constant of the air, k accounts for fringing, A is sensor area, and determining d_(p) as the total thickness of paper between sensor CIN1 and ground (total stack height), where d_(a) is the total thickness of air between the sensor and ground, so that d=d_(p)+d_(a)→d_(a)=d−d_(p) feeding one page of paper, and measuring capacitance C_(A1,1) ${{CA}\; 1},{1 = \left\lbrack {\frac{1}{{CA}\; 0} + {\frac{\left( {{dp} + {d\; 1{page}}} \right)}{ɛ\; 0\; A}\left( {\frac{1}{ɛ\; P} - \frac{1}{ɛ\; A}} \right)}} \right\rbrack^{- 1}}$ determining d_(1page); and determining page count=dp/(d1page).
 7. The method of claim 6, further comprising determining paper dielectric ∈_(P) according to: ${1/ɛ_{{pape}\underset{\_}{\underset{\_}{r}}}}\frac{{d\; ɛ_{air}} + {\left( {{nw} - d} \right)ɛ_{eff}}}{{nw}\mspace{11mu} ɛ_{eff}ɛ_{air}}$ where ∈_(eff) is the average dielectric between CIN1 and GND; n=number of pages, w=sheet thickness (d_(1page)), and stack height=nw (d_(p)); and (d) is the distance between the capacitive electrode C_(IN1) and GND.
 8. The method of claim 4, wherein the paper tray includes a capacitive sensor with inter-digitated (co-planar) capacitive E1 and ground G1 electrodes disposed in the paper tray such that paper contacts the E1/G1 electrodes, the E1/G1 electrodes configured such that during capacitive sensing a fringing electric field is substantially confined to one sheet of paper, the method further comprising: measuring capacitance CD_nopaper with the E1/G1 electrodes with no paper present; measuring capacitance CD_paper with the E1/G1 electrodes with paper present; determining paper dielectric ∈_(P) based on CD_nopaper and CD_paper, and a thickness w of one sheet of paper.
 9. A capacitve sensing system suitable for measuring paper characteristics for paper within a paper tray, comprising: at least one capacitive sensor with a capacitive electrode C_(IN1), oriented relative to the paper in the paper tray such that the paper covers C_(IN1;) an inductance-to-digital converter (IDC) configured to convert capacitive measurements from the capacitive sensor C_(IN1) into sensor data representative of at least one of the paper characteristics: paper stack height and page count.
 10. The system of claim 9, further comprising: at least a second capacitive sensor with a capacitive electrode C_(IN2), oriented relative to the paper in the paper tray such that the paper partially covers C_(IN2) in the paper width dimension; such that the IDC converts capacitive measurements from CIN1 and CIN2 into sensor data representative of at least one of the paper characteristics: paper stack height, page count, paper width.
 11. The system of claim 9, wherein the IDC converts a capacitive measurement C_(A0)=C_(IN1)-to-ground with no paper present into calibration data representative of tray thickness, sensor size/position and sensor ground plane position.
 12. The system of claim 10, wherein the IDC converts a capacitive measurement C_(A0)=C_(IN1)-to-ground with no paper present into calibration data representative of tray thickness, sensor size/position and sensor ground plane position.
 13. The system of claim 9, wherein: the IDC receives from CIN1 a capacitance measurement C_(A1)=C_(IN1) to ground; and the IDC converts the capacitance measurement C_(A1) into sensor data representative of paper stack height.
 14. The system of claim 9, wherein: the IDC receives from C_(IN1) a capacitance measurement CA1,0 ${{CA}\; 1},{0 = {\left( {{CAIR}^{- 1} + {CPAPER}^{- 1}} \right)^{- 1} = \left\lbrack {\frac{1}{{CA}\; 0} + {\frac{dp}{ɛ\; 0\; A}\left( {\frac{1}{ɛ\; P} - \frac{1}{ɛ\; A}} \right)}} \right\rbrack^{- 1}}}$ where ${CA}_{0} = {k*ɛ_{0}*ɛ_{A}*\frac{A}{d}}$ ${CAIR} = {k*ɛ_{0}*ɛ_{A}*\frac{A}{d - {dp}}}$ ${CPAPER} = {k*ɛ_{0}*ɛ_{P}*\frac{A}{dp}}$ and where ∈_(A) is the dielectric constant of the air, k accounts for fringing, A is sensor area, and the IDC converts the capacitance measurement CA1,0 into sensor data representative of d_(p) as the total thickness of paper between sensor CIN1 and ground (total stack height), where d_(a) is the total thickness of air between the sensor and ground, so that d=d_(p)+d_(a)→d_(a)=d−d_(p.)
 15. The system of claim 14, wherein: the IDC receives from C_(IN1) a capacitance measurement CA1,1, based on feeding one page of paper, and measuring capacitance C_(A1,1) ${{CA}\; 1},{1 = \left\lbrack {\frac{1}{{CA}\; 0} + {\frac{\left( {{dp} + {d\; 1{page}}} \right)}{ɛ\; 0\; A}\left( {\frac{1}{ɛ\; P} - \frac{1}{ɛ\; A}} \right)}} \right\rbrack^{- 1}}$ the IDC converts the capacitance measurements CA1,0 and CA1,1 into sensor data representative of d_(1page;) such that the sensor data is representative of page count=dp/(d1page).
 16. The system of claim 15, wherein: the IDC converts capacitance measurements from C_(IN1) CA1,0 and CA1,1 into sensor data representative of paper dielectric ∈_(P) according to: ${1/ɛ_{{pape}\underset{\_}{\underset{\_}{r}}}}\frac{{d\; ɛ_{air}} + {\left( {{nw} - d} \right)ɛ_{eff}}}{{nw}\mspace{11mu} ɛ_{eff}ɛ_{air}}$ where ∈_(eff) is the average dielectric between C_(IN1) and GND; n=number of pages, w=sheet thickness (d_(1page)), and stack height=nw (d_(p)); and (d) is the distance between the capacitive electrode C_(IN1) and GND.
 17. The system of 10, wherein: the IDC receives capacitance measurements from C_(IN1) and C_(IN2): C_(A1)=C_(IN1) to ground; C_(A2)=C_(IN2) to ground; and the IDC converts the capacitive measurements C_(A1) and C_(A2) into sensor data representative of paper width according to C_(DIFF) _(—) _(W)=C_(A1)−C_(A2), where C_(DIFF) _(—) _(W) corresponds to a percentage of C_(IN2) covered by paper.
 18. The system of claim 17: further comprising a capacitive sensor at least a third capacitive sensor with a capacitive electrode C_(IN3), oriented relative to the paper in the paper tray such that paper partially covers C_(IN3) in the paper length dimension; wherein the IDC converts capacitive measurements from C_(IN1), C_(IN2) and C_(IN3) into sensor data representative of at least one of the paper characteristics: paper stack height, page count, paper length.
 19. The system of claim 9: further comprising a capacitive sensor with inter-digitated (co-planar) capacitive E1 and ground G1 electrodes disposed in the paper tray such that paper contacts the E1/G1 electrodes, the E1/G1 electrodes configured such that during capacitive sensing a fringing electric field is substantially confined to one sheet of paper; wherein, the IDC receives a capacitance measurements: CD_nopaper with the E1/G1 electrodes with no paper present; and CD_paper with the E1/G1 electrodes with paper present; and the IDC converts the capacitive measurements CD_nopaper and CD_paper into sensor data representative of paper dielectric ∈_(P) based on CD_nopaper and CD_paper, and a thickness w of one sheet of paper. 